The present invention provides a composition and process for desensitizing lithographic printing plates which have been developed and are ready for printing. In particular, the invention provides a neutral pH, organic solvent free, phosphate free desensitizing solution which renders the non-image areas of imagewise exposed and developed plates permanently hydrophilic.
It is known in the art that it is beneficial for lithographic printing plates to be treated with a desensitizing solution after exposure and development. This is particularly advantageous for plates which are to be stored prior to use.
As the treating solution, an aqueous gum arabic solution is very widely used and it is also known to use dextrin solutions.
German Offenlegungsschrift No. 2,659,754 describes the use of aqueous polyvinyl alcohol solutions for the purpose of protecting lithographic printing plates.
U.S. Pat. No. 4,033,919 dicloses combinations of polymers of acrylamide containing containing carboxyl groups with acids, particularly phosphoric acid, which are used as hydrophilizing and desensitizing agents for printing forms.
In addition, the trend in the printing industry has been to produce plate processing compositions such as developers and desensitizers which do not contain ecologically unacceptable organic solvents and phosphates. It is also important that the developer and desensitizer be compatible since typically they are both used sequentially in automatic plate processing equipment to treat exposed printing plates. The desensitizer composition of this invention has been found to be compatible with the aqueous developer filed as U.S. patent application Ser. No. 014,969, on even date herewith and which is incorporated herein by reference.